1. Field of the Invention
Embodiments of the invention are most generally related to the field of optical signal processing. More particularly, embodiments of the invention are directed to holding beam-type, all-optical flip-flop devices and control methods thereof and, even more particularly, to remote control and polarization-independent control methods of holding beam-type, all-optical flip-flop devices as well as the devices themselves.
2. Related Art
Sequential data processing (i.e., processing with memory) is conventionally performed in the electric domain, where the latchable, output electrical signal from an electrical flip-flop is set and reset by the action of electrical control signals. More recently, sequential data processing has been realized in the optical domain. Optical flip-flops are applicable to any signal processing function that requires memory; they enable the evolution from combinational logic (for which the output signal is determined solely by the existing input signals) to sequential logic, for which the memory of past input signals also determines the state of the output signal. Examples of such applications include data-format conversion (e.g., return-to-zero (RZ) to non-return-to-zero (NRZ)), 3R regeneration (power, shape and time domains), temporal demultiplexing with bit-length stretching, switching and routing, buffering, memory, as well as the functionalities of clocks and oscillators, clock dividers, latches, resisters, counters, adders, and transistors. Some advantages of sequential processing methods and apparatus in the optical domain include high speed of operation (e.g., bandwidth exceeding 1 GHz), EMI/EMP resistance, the absence of the need for optical-to-electronic conversion, which allows control signals to originate directly from digital optical data and communication networks, the evolution of Photonic Integrated Circuits (PICs) that may provide a substantial increase in reliability, functionality, and reconfigurability, as well as substantial reduction in power usage, physical footprint, and hardware and deployment costs.
Flip-flop devices that operate on the basis of optical bistability such as dispersive bistability, absorptive bistability, mixed dispersive and absorptive bistability, and four-wave-mixing bistability, for example, utilize a continuous-wave (cw) holding beam. Control (SET, RE-SET) signals may operate by various processes to change the power of the holding beam or the required switching power threshold of the bistable medium.
SET and RE-SET operation in a resonant-type semiconductor optical amplifier (RT-SOA)-based flip-flop was previously reported using an optical modulator that increased and decreased the holding beam power, but did not use optical control signals. A RT-SOA-based flip-flop provides a desirable non-linear bistable medium by reason of its sub-millimeter size, optical gain, and strong nonlinearity. SOAs also have the potential for integration into PICs and for supporting switching at 40 Gb/s and above. A direct optical RE-SET technique was previously reported using a closely tuned (within 0.008 nm) optical beam that interfered with the holding beam within the bistable RT-SOA. Remote optical RE-SET was previously reported using cross-phase modulation (XPM) to reduce the holding-beam power passing through a dye-filled Fabry-Perot cavity. Optical SET and RE-SET techniques have also been demonstrated using widely detuned (>30 nm) pulses that entered the bistable RT-SOA and varied the optical-power hysteresis by means of cross-phase modulation (XPM). The interested reader is directed to the following for further information: U.S. Pat. No. 6,456,417; Maywar, Nakano, and Agrawal, Robust optical control of an optical-amplifier-based-flip-flop, OPTICS EXPRESS 6, No. 3, pp 75-80 (January 2000); and, Maywar, Nakano, and Agrawal, All-optical hysteresis control by means of cross-phase modulation in semiconductor optical amplifiers, J. OPT. SOC. AM. B 18, No. 7, pp 1003-1013 (July 2001), the subject matters of which are incorporated by reference herein in their entireties to the fullest extent allowed by applicable laws and rules.
In addition to in-line RT-SOA bistable media for optical flip-flops, recent research has focused on bistable vertical cavity semiconductor optical amplifiers (VCSOAs), which are advantageous for some applications due to their compact, symmetrical-waveguide, surface-emitting design.